Tubes in multi-tube heat transfer devices usually require cleaning on a regular basis. For the most part, this is accomplished by lancing, a manual or partly-mechanized process in which the nozzle of a flexible or rigid lance, emitting high velocity jets of water, is passed through each tube. When manually performed, lancing is an arduous, dirty and dangerous task. Large volumes of effluent are generated, which often contains toxic substances. The cleaning jets of high velocity water are capable of inflicting severe injury. Environmental regulations governing the disposal of effluent are strict and the production of minimal effluent volume is thus desirable. A rigid lance must be as long as a tube to be cleaned if cleaning is to be effected in a single pass. Particularly in long tubes, manipulation of lances generally and the head space required for deployment of rigid lances often represent a considerable problem.
Australian Patent Applications Nos. 18,165/83 and 32,187/84 disclose a method of cleaning multi-tube heat transfer devices such as boilers, condensers and heat exchangers. In the method disclosed therein, a tube is cleaned by propelling into and through it a projectile made to travel at high speed by means of a more or less instantaneous release of a pressurized liquid cleaning medium. In most applications, it is usual for the said cleaning medium to be water with typical working pressures ranging from 1,000 to 10,000 P.S.I.
The aforementioned patent specifications use the term "pig" to describe the said projectile, however, this terminology is considered not to be altogether appropriate as the cleaning method differs from the well-known pigging cleaning method in material ways. In pigging, the pig is constructed so that its outer or forward surfaces, or auxiliary scraping, abrading or brushing elements attached thereto, are kept firmly in mechanical contact with the inner surfaces of a pipe or tube being cleaned. The pig is drawn or propelled slowly through the tube or pipe, the said surfaces or auxiliary elements effecting the cleaning in a direct, mechanical way. In the present method, the projectile is made to have a light sliding fit in the bore of a tube to be cleaned or of a diameter to approximate the reduced lumen created within the tube by a heavy accretion of contaminant material. Thus it cannot itself mechanically remove contaminant material.
In operation, where a tube is completely blocked, the release of the pressurised cleaning medium causes the projectile to accelerate rapidly into the tube, whereupon the projectile and its propelling column of liquid cleaning medium are arrested violently by the projectile's coming into contact with the deposit of contaminant material. It is believed that the resultant water hammer effect causes an energetic shock to pass along the length of the tube, breaking the bond between its inner wall and the contaminant material. In some types of monolithic crystalline deposit, the said shock causes the contaminant material to revert to a granular or particulate form. The loosened contaminant material is ejected from the tube by the projectile, its progress through the tube being assisted by pressure pulses in the following column of cleaning medium. The said pressure pulses are generated by the action of the pistons of the multi-cylinder positive displacement type pump preferably employed to pressurized the said cleaning medium.
Where the progress of the projectile through the reduced lumen of a tube is impeded by a partial deposit of contaminant material, it is believed that a flow of pressurized cleaning medium may pass through the annular space between the projectile and the contaminant material, emerging at the front of the projectile in the form of a high velocity annular jet. It is thought that the jet serves to erode away from ahead of the projectile deposits of contaminant material, facilitating the progress of the projectile through the tube.
Where a projectile is able to pass through a tube unimpeded, as is the case in tubes containing contaminant material in the form of light, laminar deposits, it is believed that an energetic cleaning effect is generated in the cleaning medium in the wake of the said projectile and that this is related to the speed of the projectile. This effect, the nature of which is not yet fully understood, appears to involve the action of cavitation or a cavitation-like process. In all cases where a projectile is free to progress through a tube, it is not necessary for the projectile to contact the walls of the tube or the contaminant material adhering thereto for the said cleaning effect to be generated.
The aforementioned patent applications disclose a launcher which comprises a power cylinder in one end of which is slidably mounted a barrel member, and a piston movable within the said cylinder, said piston being fixed to a cleaning medium duct made coaxial with it and the said cylinder such that forward movement of the piston causes the said cleaning medium duct to pass up the barrel member of the launcher, pushing a projectile from a storage magazine mounted upon the barrel member and causing the projectile to travel up the said barrel and into a tube to be cleaned adjacent the end of and collinear with which the muzzle of the said barrel member has been positioned. In the final part of its forward movement, the said piston contacts the inner end of the said barrel member, causing the barrel member to move bodily forward in its said slidable mounting in the end of the said cylinder, against the pressure of a restoring spring, and press its muzzle firmly against the end of the tube to be cleaned. A flow of high pressure cleaning medium is then released through the said cleaning duct propelling the projectile through the tube and generating a cleaning effect.